Automatic sleep stage classification allows for real-time sleep staging and remote monitoring of subjects. One of the parameters of interest derived from sleep stages is called sleep onset latency (SOL) which is an indicator in the evaluation of sleep complaints and also in the evaluation of sleep disorders such as insomnia and circadian rhythm disorders. The SOL basically measures the amount of time elapsed between the instant when the subject wishes to sleep until the moment when that subject eventually falls asleep.
It is known to determine SOL by classifying periods of wake and sleep and then, for instance, determining SOL as the amount of time before a certain criterion is determined, for instance, until the first occurrence of three consecutive epochs classified as sleep.
Actigraphy devices are employed to detect periods of wake and sleep based on body movements of the subject. While these body movements are indicative for brief periods of awakening during sleep since these occur together with body movements, this is not always valid at the beginning of the night, in particular for determining the SOL. Since subjects, particularly subjects suffering from a long SOL, try to lay as still as possible, actigraphy based SOL estimates underestimate the SOL.
WO 2010/140117 A1 discloses a method for the automatic assessment of the presence/severity of the sleep problem and its exact nature. The assessment is based on qualitative information about sleep patterns, insomnia-related factors and daytime consequences, as well as quantitative information about sleep patterns measured by a sensor. Multiple sensors provide a sensor system signal incorporating the sleeping activity data, wherein the sleeping activity data includes at least heart rate data, respiratory rate data and patient body movement data.
The article “Sleep Assessment using a Passive Ballistocardiography-Based System: Preliminary Validation” by D. Mack et al., IEEE, pages 4319-4322 (2009) discloses a system for non-invasively analyzing physiological signals, which uses resilient pads placed on a bed to record minute movements associated with cardiac and respiratory functions, i.e. ballistocardiography, which is compared to actigraphy in distinguishing between sleep and wake periods.
The article “Comparison of actigraphy immobility rules with polysomnographic sleep onset latency in children and adolescents” by L. Meltzer et al., Sleep Breath, pages 1415-1423 (2015) evaluates different criteria of immobility as a measure of sleep onset latency in children and adolescents for determining the validity of actigraphy as an estimate of sleep-wake patterns.